An imaging system capable of recording images in a narrow range of light frequencies can be useful when imaging a scene having various components which emit or absorb radiation at specific, typically different, frequencies. Using such a monochromatic imaging system, spatial and spectral information can be recorded about a particular component in the scene. Some imaging applications require an imaging system which can not only record images in a narrow range of light frequencies but which has high sensitivity, high spectral resolution, and high operational data rates. Such applications include aeronomic research, military, and environmental. For aeronomic research applications, such a monochromatic imaging system can be used to detect spatial and spectral information useful in determining specific atmospheric constituents. For military applications, such a system can aid in the identification and location of chemical agents, equipment, and personnel. In environmental applications, such a system can monitor the transportation of hazardous substances from waste sites and monitor pollutants released from manufacturing installations.
Some known monochromatic imaging systems and methods use conventional bandpass or interference filters. While such bandpass or interference filters offer high throughput and thus good sensitivity to weak signals, only moderate (1 to 2% of center wavelength) spectral resolution is possible and only discrete frequencies can be observed. Also, because multiple bandpass or interference filters typically are employed, the physical size and mechanical complexity of a system including such filters is increased as compared to a system which does not include such filters.
It also is known to use circular variable filter (CVF) spectrometers and linear variable filter spectrometers in monochromatic imaging applications. While these spectrometers can be continuously tunable in wavelength, they typically deliver only moderate spectral resolution and achromatic images. If high spectral resolution is required, only a small portion of a filter wheel associated with the spectrometer can be illuminated because different portions of the wheel pass different frequencies. Decreased throughput and sensitivity to weaker signals results when only a small portion of the wheel is illuminated.
Monochromatic imaging also can be achieved with known grating spectrometers which use toroidal optics. These grating spectrometers typically are designed specifically to achieve high spectral resolution imaging. These grating spectrometers, however, provide low throughput since incoming light must be passed through an entrance slit and imaging occurs only along one dimension. Because spectral dispersion occurs in the other dimension at an image plane, two-dimensional imaging can be achieved by recording a scene twice with the spectrometer's orientation changing by 90 degrees between recordings, but this method tends to be impractical and inappropriate for rapidly changing scenes.
Some known monochromatic imaging systems use Fabry-Perot interferometers (FPIs). In general, advantages of a FPI include high spectral resolution and high throughput which can be due to the large input aperture typically associated with a FPI. An FPI typically is operated in "high order" meaning that the spacing of mirrors in the FPI is much greater than the transmission wavelength of the FPI. When operated in high order, the FPI typically produces, at an image plane, a discontinuous image which can be a series of concentric circles with or without a spot at the center. Only information associated with the portions of an imaged scene which are coincident with the concentric rings and the central spot are present in the discontinuous image. Space between the rings contains no information about the imaged scene. To obtain all information associated with the imaged scene, a continuous image must be constructed from the discontinuous image. Algorithms for constructing a continuous image from the discontinuous image are known. A computer typically is employed to execute such algorithms.